Added ability to change public directory

If you didn't use `public` as public directory, version would fail, no matter what because it wass hard coded. This change will allow you to change it.

If you are using `public_html` instead of `public`, you can with this change use

    .version('css/app.css', 'public_html', 'public_html');

This will set the base path to `public_html` (third argument). Second argument would tell the build directory to be `public_html/build`.